When highly accurately observing an optical frequency, the technique of heterodyne detection is employed. With heterodyne detection, light to be observed is made to interfere with other light to detect the electric signal of the generated optical beat frequency. The band of light which can be observed that can be used for heterodyne detection is limited by the frequency band of the light receiving element to be used for the detection system and is about tens of several GHz.
Meanwhile, as a result of the development of optoelectronics in recent years, the need for extending the band of light which can be observed has been intensified for the purpose of optical control for frequency multiplex communications and frequency observations of widely distributed absorption line.
Broad band heterodyne detection systems using an optical frequency comb generator have been proposed (see, for example, Patent Document 1: Jpn. Pat. Appln. Laid-Open Publication No. 2003-202609) to meet the need for extending the band of light which can be observed. Optical frequency comb generators are adapted to generate comb-shaped sidebands appearing at regular frequency intervals. The frequency stability of the sidebands is substantially equal to that of incident light. It is possible to set up a broad band heterodyne detection system of several THz where the generated sidebands and light to be observed are subjected to heterodyne detection.
FIG. 1 of the accompanying drawings schematically illustrates the principle of structure of a known optical frequency comb generator 3 of the bulk type.
Referring to FIG. 1, an optical resonator 100 including an optical phase modulator 31 and reflectors 32, 33 arranged opposite to each other with the optical phase modulator 31 interposed between them is used in the optical frequency comb generator 3.
The optical resonator 100 causes light Lin that enters it by way of the reflector 32 with a small transmission factor to resonate between the reflectors 32, 33 and allows part of light Lout by way of the reflector 33. The optical phase modulator 31 is formed by using electrooptic crystal for optical phase modulation of changing the refractive index by applying an electric field and adapted to modulate the phase of light passing through the optical resonator 100 according to the electric signal of frequency fm applied to the electrode 36.
With the optical frequency comb generator 3, it is possible to modulate the phase of light deeper by tens of several times than ever by using an electric signal that is synchronized with the time necessary for light to make a round trip in the optical resonator 100 and driving it to enter from the electrode 36 into the optical phase modulator 31 if compared with light that is made to pass through the optical phase modulator 31 only once. With this arrangement, it is possible to generate several hundreds of sidebands of higher orders. Then, all the frequency intervals fm of adjacent sidebands are equal to the frequency fm of the input electric signal.
Known optical frequency comb generators are not limited to the above described bulk type. For example, a waveguide type optical frequency comb generator 200 including a waveguide as shown in FIG. 2 of the accompanying drawings is also feasible.
Referring to FIG. 2, the waveguide type optical frequency comb generator 20 includes an waveguide type optical modulator 200. The waveguide type optical modulator 200 includes a substrate 201, a waveguide 202, an electrode 203, an incidence side reflection film 204, an emission side reflection film 205 and an oscillator 206.
The substrate 201 is typically formed by cutting a large crystal of LiNbO3 or GaAs with a diameter of 3 to 4 inches grown by a pulling method into a wafer. The surface of the substrate 201 produced by cutting is then subjected to a mechanical polishing process and/or a chemical polishing process.
The waveguide 202 is provided to propagate light. The refractive index of the layer of the waveguide 202 is set to be higher than that of any other layer such as the substrate 201. Light that enters the waveguide 202 is propagated through the waveguide 202 as it is totally reflected by the interface thereof. Generally, the waveguide 202 can be prepared by diffusing Ti atoms in the substrate 201 or by depositing Ti atoms on the substrate 201 by epitaxial growth.
Note that an LiNbO3 crystal type optical waveguide may be used as the waveguide 202. An LiNbO3 crystal optical waveguide can be formed by diffusing Ti atoms on the surface of a substrate 201 mainly made of LNbO3. When preparing an LiNbO3 crystal type optical waveguide, firstly a photoresist pattern is formed on the surface of the substrate 201 and then Ti atoms are deposited. Subsequently, the photoresist is removed to produce Ti micro-wires having a width of microns. Thereafter, Ti atoms are thermally diffused in the substrate 201 by heating the Ti micro-wires.
As Ti is thermally diffused in the substrate 201 of LiNbO3, light can be confined to the region where Ti is diffused as the region shows a refractive index higher than that of any other region. Thus, a waveguide 202 that can propagate light through the region where Ti is diffused is formed. Since an LiNbO3 crystal type waveguide 202 prepared in a manner as described above has electrooptic effects, it is possible to change the refractive index by applying an electric field to it.
The electrode 203 is typically made of a metal material such as Al, Cu, Pt or Au and adapted to drive and input an electric signal of frequency fm into the waveguide 202. The direction of propagation of light agrees with the direction of progression of the modulation electric field. The speed of light propagating through the waveguide 202 may be made to agree with the speed of the electric signal propagating on the electrode 203 by adjusting a width and thickness of the electrode 203. With this arrangement, it is possible to maintain the phase of the electric signal relative to light propagating through the waveguide 202.
The incidence side reflection film 204 and the emission side reflection film 205 are provided to resonate light that enters the waveguide 202 by reciprocatingly reflecting light passing through the waveguide 202. The oscillator 206 is connected to the electrode 203 to supply an electric signal of frequency fm.
The incidence side reflection film 204 is arranged at the light receiving side of the waveguide type optical modulator 200 and receives light of frequency ν1 from the light source. The incidence side reflection film 204 reflects light that is reflected by the emission side reflection film 205 and passed through the waveguide 202.
The emission side reflection film 205 is arranged at the emission side of the waveguide type optical modulator 200 and reflects light that is passed through the waveguide 202. It also emits light that is passed through the waveguide 202 to the outside at a predetermined ratio.
Since the electric signal synchronized with the time necessary for light to make a round trip in the waveguide 202 is driven and input from the electrode 203 to the waveguide type optical modulator 200 of the waveguide type optical frequency comb generator 20 having the above-described configuration, it is possible to modulate the phase of light deeper by tens of several times than ever by using an electric signal that is synchronized with the time necessary for light to make a round trip in the optical phase resonator 111 and driving it to enter from the electrode 203 into the waveguide type optical modulator 200 if compared with light that is made to pass through the optical phase modulator 111 only once. With this arrangement, it is possible to generate broad sidebands like the above-described bulk type optical frequency comb generator 10. Then, all the frequency intervals fm of adjacent sidebands are equal to the frequency fm of the input electric signal.
The waveguide type optical frequency comb generator 20 is characterized by a small interacting region of light and an electric signal. Since light is confined in the waveguide 202 of dimensions in the order of microns having a refractive index higher than that of the surroundings and propagated, it is possible to locally raise the electric field intensity in the waveguide 202 by fitting the electrode 203 at a position close to the pole of the waveguide 202. Therefore, the electrooptic effects obtained in the waveguide 202 are greater than those of the waveguide of the bulk type optical frequency comb generator 3 so that the waveguide type optical modulator 200 can modulate light with less electric power.